This invention relates to planar heat generating devices which are employed in manufacturing chemicals, processing semiconductors and plating, or in heating corrosive fluids in laboratories, and more particularly to a heat generating device which uses heat generating units made of positive temperature characteristic resistance material.
In one example of a conventional planar heat generating device for heating fluid, metal resistors of nichrome or techrome are employed as heat generating units, and these heat generating units are directly covered with heat resisting synthetic resin. In another example, sheathed heaters of such metal resistors are buried in a heat radiating metal plate, which is covered with a heat resisting synthetic resin.
In such conventional heat generating devices, the temperature of the metal parts must be kept lower than the melting point or the deterioration point of the synthetic resin. Accordingly, it is essential to set the electrical capacity of the heat generating units relatively low. Therefore, with the conventional heat generating devices, it takes a relatively long time to raise the temperature of the fluid to a desired value. Especially when a fluororesin is employed, the characteristic of the heat generating units cannot be fully utilized, because the fluororesin is low in heat conductivity although it is excellent in heat resistance and corrosion resistance.
In order to overcome these drawbacks of conventional heat generating devices, a heat generating device has been proposed in which a heat-sensitive sensor is mounted on a heat radiating metal plate to protect the cover of heat resisting synthetic resin and to increase the electrical capacity, and temperature control is effected below the melting point or deterioration point of the synthetic resin. However, the device is still disadvantageous in that only the temperature of the heat radiating metal plate is abruptly raised to operate the heat-sensitive sensor for temperature control, and the synthetic resin cover layer is low in heat conductivity, and therefore it takes a long time to increase the temperature of the fluid to a desired value.
Furthermore, sometimes the synthetic resin cover layer is peeled off the heat radiating metal plate by the heat generated. If this trouble occurs, the heat conducting efficiency is lowered or becomes non-uniform, and sometimes it is impossible to raise the fluid temperature to a desired value.
Since it is necessary to connect lead wires to the heat-sensitive sensor, the heat generating device is intricate in construction. In addition, during the use of the device in a fluid, it is necessary to control the fluid temperature and the heat generating units. Thus, handling the device is rather troublesome.